Chromosomal Inheritance

Chromosomal Theory of Inheritance

• Chromosomes contain the units of heredity (genes)

• Pair chromosomes segregate during meiosis, each sex cell has half of the number of chromosomes found in a somatic cell. (Mendel’s law of segregation)

• Chromosomes assort independently during meiosis (Mendel’s law of independent assortment)

• Each chromosome contain many different genes

Sex Chromosomes

• Sex chromosomes (X and Y) vs. autosomes (chromosomes 1-22), Sex cells and somatic cells.

• Homogametic sex -- that sex containing two like sex chromosomes. In most animals species these are females (XX). Each egg only contain one X chromosome.

• Heterogametic sex --- that sex containing two different sex chromosomes . In most animal species these are XY males. Each sperm will contain either an X or Y. Therefore the father determines whether the offspring is a boy or a girl (50/50 chance)

Sex Linkage

• XA = Locus on X chromosome • XX females

– XA XA, XaXa - homozygotes – XA Xa – heterozygote (carrier)

• XY male – XA Y, XaY – no carriers in males, therefore they are more

susceptible to x-linked traits.

Red/white eye color in Drosophila

• In females: – XR XR , XR Xr = red-eye female – Xr X r = white-eyed females

• In males:– XR Y = red-eye male – Xr Y = white-eyed male

Examples of Sex Linked Traits

• Hemophilia - Recessive

• Red-Green Color Blindness - Recessive

• Muscular Dystrophy - Recessive

• Fragile X syndrome - Dominant

Nondisjunction

abnormal number of autosomal chromosomes when chromosomes fail to separate during replication.

• 2n – 1 = monosomic

• 2n + 1 = trisomic

Nondisjunction

Nondisjunction - Examples

• Down's -- trisomy 21 mean life expectancy 17 years. Short in stature, round face and mental retardation

• Patau's -- trisomy 13 mean life expectancy 130 days

• Edward's --- trisomy 18 mean life expectancy a few

weeks

Sexual Determination - Males

Single Y = male, so XXY, XYY, XXXY all male

• Klinefelter Syndrome – XXY or XXXY. Male due to Y chromosome, Testes and prostrate underdeveloped, some breast formation, no pubic or facial hair, subnormal intelligence.

• Jacob’s Syndrome – XYY. Males are usually taller than average, and tend to have speech and reading problems

Sexual Determination - Females

• Turner’s Syndrome – X0. Female with bull neck, short stature, nonfunctional ovaries, no puberty

• Metafemale – 3 or more X chromosomes. No apparent physical abnormality except menstrual irregularities.

Chromosomal Mutation

• Permanent change in chromosome structure.

• Caused by exposure to radiation, organic chemicals, viruses, replication mistakes.

• Only mutations in sex cells are passed onto the next generation.

Structural Changes in Chromosomes

• Inversion – occurs when a chromosome segment turns around 180 degrees.

• Translocation – is movement of chromosomal segments to another non-homologous chromosome

• Deletion – occurs when a portion of the chromosome breaks off.

• Duplication – when a portion of a chromosome repeats itself.

Deletions and Duplications

Inversions and Translocations

Linkage

• When genes are on the same chromosome, they are called linked. They can show departures from independent assortment

• If genes on the same chromosome are sufficiently far apart, they can segregate independently through

crossing over.

Gene Mapping

• By studying cross-over (recombination) frequencies of linked genes, a chromosomal map can be constructed– Distant genes are more likely to be

separated by crossing-over than genes that are closer together.

– Each 1% of recombination frequency is equivalent to 1 map unit

Crossing Over Produces Recombinations

Constructing Gene Maps

• Crossing over frequencies can be used to construct gene maps. For example, – Crossing over frequency of genes A and B

is 3%, genes B and C is 9% and genes A and C is 12%.

3 mu 9 mu

A B C

Human Genome Project• Map of the all of the genes on the

human chromosomes.

Pedigree Analysis

Modes of Inheritance• Autosomal dominant allele

[e.g., Huntington's Disease, brown eyes] – A phenotype associated with an

autosomal dominant allele will, ideally, be present in every individual carrying that allele. It will be present in close to 50% of the individuals.

– Affected children usually have affected parents

– Two affected parents can produce an unaffected child

– Both males and females are affected equally.

Modes of Inheritance• Autosomal recessive alleles [silent

carriers] – albinism, cystic fibrosis, certain types of

hemophilia, Tay-Sachs disease, PKU, blue eyes.

– A pedigree following a trait associated with an autosomal recessive allele is often marked by a skipping of generations. That is, children may express a trait which their parents do not.

– In such a situation, both parents are heterozygotes, also known as silent carriers.

– Close relatives who reproduce are more likely to have affected children.

– Both males and females will be affected with equal frequency

– A low number of individuals normal affected

Modes of Inheritance

• Sex-linked dominant alleles [sex linkage]

– A sex linked dominant allele has a variation on the pattern displayed by autosomal dominant alleles. That is:

• one-half of the offspring of an afflicted heterozygote female will be similarly afflicted (gender independent).

• only the female progeny of males will be afflicted (because the male donates an X chromosome to his female progeny).

– As with any sex-linked allele, males can pass the allele only on to their daughters, not their sons.

Mode of Inheritance• Sex-linked recessive alleles

– red-green color blindness, certain types of hemophilia.

– More males affected than females

– An affected son can have parents who have the normal phenotype

– For a female to have the characteristic, her father must also have it and the mother must be a carrier.

– If a woman has the characteristic all her sons will have it

– The characteristic often skips a generation from grandfather to grandson